1. Field of the Invention
This invention relates generally to screen printing on a laminated assembly of low temperature co-fired ceramic (LTCC) material, and more particularly to a method and apparatus for screen printing gold onto fired LTCC substrates of microwave transmit/receive (T/R) modules to provide a solderable surface for side connectors, ring frames, and heat sinks that package the ceramic.
2. Description of Related Art
In forming an LTCC substrate, such as shown in related application Ser. No. 10/718,805, typically a number of LTCC tape layers are initially divided into multiple stacks of tape layers, e.g., four stacks or sections, based on a final cavity routing configuration of each stack. One of the stacks preferably comprises a set of layers for the formation of a ceramic ring frame. Each stack is then separately tack laminated, followed by cavity patterns being machined therein using a router device whereby a large number of layers are routed in a single pass, forming cavities having side walls which are relatively smooth and straight as opposed to the known prior art method of cutting individual layers with a laser, for example, which results in the formation of jagged side walls. Following cavity routing, all of the stacks are laid up on a base plate including a set of tooling pins for providing alignment of the stacks. The lay up is completed with a placement of a top mylar template followed by a copper template. The assembly is next covered with an expandable latex sheet that has been lightly coated with material for aiding the removal of the latex sheet without metal lift-off after lamination. The assembly is then placed in a lamination fixture and isostatically laminated for a specific period of time at a predetermined pressure and temperature, typically 4000-5000 psi at 72° C. for 15 minutes, resulting in a structure in the form of laminated panel. The panel is then allowed to cool, removed from the fixture where it is green cut to remove tooling holes and then fired with a designated firing profile. A plurality of mutually identical newly designed LTCC substrates for a T/R module are then diced, i.e. cut, from the panel and post fire printed as required.
Due to the design of conventional screen printers, e.g., the well known MPM SP-1500 printer, the printing of post-fired LTCC substrates incorporating the above noted design was limited to a certain range of the overall thickness of the substrate or board. For instance, the allowable thickness of the substrates were limited to less than 1″ maximum. Substrates with a thickness greater than 1″ were not able to access the print position inside of the standard screen printing machine due to the fact that the printer head was in the way.
Accordingly, screen printing LTCC substrates in the post-fired condition require a certain type of unique fixturing of the substrate on a screen printer for accurate and consistent placement of the printed pattern.